RC Snubber Calculator (Damp Ringing / Reduce EMI)
Turn a scope capture into RC snubber values: estimate parasitic L/C from ringing frequency, pick Csnub, and get a starting Rsnub plus loss estimates.
Quick start examples
Examples load plausible numbers to show the workflow. Use your real scope measurements.
Inputs
How to measure
Snubber choice
A common starting point is Csnub = 2× to 5× the estimated parasitic capacitance (Cpar). Larger Csnub damps more strongly but increases switching loss.
Try 2 to 5 for a first prototype.
Example: 30% → try 0.7×R to 1.3×R.
Loss estimate (optional)
This is a first-order estimate assuming the snubber capacitor is charged/discharged each cycle. Real loss depends on waveform, placement, and whether Csnub sees the full voltage swing.
Results
Suggested snubber (starting point)
Csnub
—
Rsnub
—
—
Rsnub starts from the common rule-of-thumb R ≈ √(Lpar / Csnub). Sweep around it.
Estimated parasitics
Cpar: —
Lpar: —
Expected resonance
With Csnub: —
RC corner: —
Loss estimate (snubber resistor)
P (est.): —
IRMS (est.): —
Suggested rating: —
Check pulse/voltage ratings and temperature rise.
Sanity checks
How to measure (fast workflow)
- Probe correctly: use a ground spring or coax tip (long ground leads create fake ringing).
- Measure the baseline ringing frequency f0 right after the edge.
- Solder a known capacitor Ctest at the same physical location where the snubber will go and measure f1.
- Choose a multiplier (for example 2× to 5× Cpar) and start with the suggested R.
- Validate: overshoot reduction, EMI improvement, and resistor temperature. Then sweep R around the suggested value.
Keep your probe setup identical between f0 and f1. The probe adds capacitance/inductance, so consistency matters more than absolute accuracy.
A snubber is a damping tool. If the root cause is a large high-di/dt loop, fix layout first (tight loops, short returns, proper decoupling), then add a snubber if needed.
Placement checklist (where most snubbers fail)
- Place the RC right at the ringing node (smallest possible loop area).
- Use a dedicated return via next to the snubber capacitor/return pin.
- Do not route the snubber through a long trace (it becomes an inductor and stops damping).
- Pick a capacitor with SRF above the target ringing (otherwise ESL dominates and damping collapses).
- Start conservative: smaller Csnub, then increase if needed while watching dissipation.
FAQ
Series RC vs capacitor alone?
A capacitor alone can shift the resonance but may not damp it. Adding a resistor turns energy into heat and reduces Q, which is what suppresses ringing.
My measured f1 is not lower than f0 — why?
Common causes are probe-induced artifacts, Ctest not actually at the ringing node, or the ringing being dominated by a different mode than the one you are targeting.
Is an RCD clamp the same as an RC snubber?
No. An RCD clamp is typically used to limit peak voltage by catching leakage energy (common in flybacks). A simple RC snubber is primarily a damping network for a resonance.